There are various systems available for preventing the release of fuel gas unburned from or burner. A gas valve with a magnetic control can be provided in combination with a temperature responsive element, for example, a bimetallic element or thermocouple, to generate an electric current while a gas flame remains lit to supply the magnetically operated valve and maintain the spring loaded valve open as long as the gas flame heats the sensor tip of the thermal element. Should the gas flame be extinguished at the burner the temperature of the flame is no longer detectable by the tip of the sensor and the gas valve can automatically close by the action of the spring to shut off the supply of gas to the burner.
This system has the drawback that the gas valve is operated via a stem which is depressed manually and must be held in until the electric current generated by the thermal element is sufficient to retain the magnetic valve in the open position. In most instances, it is necessary to depress the stem or plunger for up to 10 seconds before the valve will be retained in its open state.
Another drawback is that, upon extinction of the flame at the burner, the closure of the gas valve may take up to 90 seconds so that over this period of time unburned gas can be released into the atmosphere from the burner. Furthermore, the way in which the thermoelement must be mounted on the burner can be comparatively expensive and, with time, the sensor tip of the thermal element can be damaged or soiled and can become unusable.
Another approach, with a burner having no automatic shutoff is to provide for ionization ignition by a spark plug at the burner. An electric amplifier measures the current between the spark plug at the burner and a burner covered and the degree of ionization thus measured can indicate whether the gas flame is present or if a gas flame is absent where a gas flame is found to be absent by this system, the spark plug is triggered to reignite the burner. In this case, if a train of ignition pulses is transmitted to the spark plug after the gas flame has been extinguished and the flame is not reignited, unburned gas continues to flow out of the burner during the period. The gas supply then can be cutoff only by manual operation of the gas cook. In the case of a defective burner, removal or loss of the burner cover or even partial blockage of gas outlet openings, the spark generating igniter can fire without igniting the gas so that the gas can flow unburned from the burner.